Reuben,
The insulation resistance of the PVC will decrease in your rainy (monsoon) season. Do you dry the pellets to -40C dew point before extrusion. Are you using the correct (minimum) amount of color masterbatch with gravimetrics??
PVC is a mixture of materials (clays and such)and many companies manufacture their own PVC. If you purchase your PVC, get the supplier to tell you what the insulation resistance should be.
With regards to the XLPE (Moisture cure?), get the manufacturer to tell you what the insulation resistance should be when using a specified minimum amount of color masterbatch or natural compound. Make sure you always mix your compounds under gravimetric control.
Regards
Peter J. Stewart-Hay
Principal
Stewart-Hay Associates
www.Stewart-Hay.com
519 641- 3212